1. Field of the Invention
The present invention relates to contact lenses for correcting presbyopia, and more particularly a system of lenses wherein for every add need, a disparate pair is provided resulting in improved visual performance. More specifically, the contact lens pair will have a level of disparity between the two eyes that is controlled to achieve better visual performance and that is not objectionable to the wearer.
2. Discussion of the Related Art
As individuals age, their eyes are less able to accommodate, or bend their natural or crystalline lens, to focus on objects that are relatively near to the observer. This condition is known as presbyopia. More specifically, when an individual is born, the crystalline lens is pliable which makes it capable of a high degree of accommodation. As the individual ages, the crystalline lens gradually becomes more rigid and thus less able to accommodate. Similarly, for persons who have had their natural or crystalline lens removed and an intraocular lens or IOL inserted as a replacement, the ability to accommodate is absent. Although the intent of an accommodating IOL is to address this potential shortcoming, current accommodating IOL designs and concepts are relatively new and continue to evolve.
Among the methods used to correct for the eye's failure to accommodate is a method known as mono-vision in which, in most cases, a contact lens for correction of distance vision is utilized in the lens wearer's dominant eye, which is known to predominate for distance vision, coupled with a second contact lens for correction of near vision being utilized in the non-dominant eye. Mono-vision provides for both near and distance vision while allowing the brain to compensate as how the images should be construed. Another known method for correction of presbyopia is to use bifocal or multifocal contact lenses in both of the individual's eyes. There are many forms of bi-focal or multi-focal contact lenses for the correction of presbyopia. These design forms include concentric rings and aspheric designs, both of which may be designed for center distance or center near. All of these designs function by providing a range of powers within the pupil of the eye. For example, a concentric ring design may have a central ring that provides powers that are nominally equal to the power required to correct the distance vision of the subject, an adjacent ring that provides near powers, and an outer ring that also provides distance powers. There may also be fitting strategies to address intermediate vision demands between near and far distances, for example, computer screen viewing. Use of bifocal or multifocal lenses in both eyes results in a reduction of image contrast and resolution compared to mono-vision but usually maintains binocularity. Yet another method of treating presbyopia is to place a bifocal or multifocal lens in one eye and a single vision lens in the other eye. The disadvantage in using this method is in the large number of lenses that must be considered in order to provide the individual with satisfactory lens performance and the limitation of binocularity at near distance.
Various classes of contact lens and intra-ocular lens designs are available for the treatment of presbyopia. One solution for presbyopic patients, as briefly set forth above, is to provide them with what is commonly referred to as monovision. With monovision, a single vision lens corrected for best distance vision is placed in the dominant eye. In the non-dominant eye, the single vision lens is fit with power that is plus in power relative to the refraction that gives the best distance vision by an amount equal to the add need of the patient. For example, for a patient that has a distance refraction of −3.0 D in both eyes and a +2.0 D add need, the dominant eye is fit with a −3.0 D spherical lens and the non-dominant eye is fit with a −1.0 D spherical lens. The term “add need” set forth above refers to the increase in power relative to the best distance correction power required to provide the presbyopic patent with near vision at a working distance of 40 cm.
A problem associated with monovision is that once the add need is greater than +1.75 D, many patients cannot tolerate the visual disparity between the two eyes and a decrease in binocularity. Eye care professionals typically define disparity simply as the difference in power relative to distance refraction between the two eyes; accordingly, in the above example, the disparity in power is 2.0 D. Binocularity is defined as the ability to focus on an object with both eyes and create a single stereoscopic image.
For patients with astigmatism, the vision compromise is generally even greater. Also, manufactures are less likely to even provide design for astigmatic presbyopia because of the large number of SKUS required and the problems associated therewith.
Accordingly, it may be readily seen that current lens systems for presbyopic patients inadequately meet or address the patient's needs.
As used herein, a lens system refers to a system of lenses, normally two or three unique designs that are required to meet the add needs for a presbyopic population with add needs ranging from 0.75 to 2.5 D or more. As offered for sale, this lens system must also include a recommended fit guide which tells the eye care professional which lens or lenses from the lens system are fit to which eye (dominant/non-dominant) in order to provide the best vision possible. Accordingly, eye care professionals are concerned about giving the patients the best possible vision and also that the system be easy to fit and the number of trial lenses that are required to be stored in their office also be kept to a minimum. As the case with patients, it may be readily seen that current lens systems for patients with presbyopia inadequately meet the eye care professionals' needs.